A grain oriented electrical steel sheet is used mainly as an iron core material for a transformer and other electrical equipment and is excellent in magnetic properties, such as excitative and iron loss properties. The magnetic flux density, B.sub.8, at a magnetic field strength of 800 A/m is usually used as a numeric value for expressing the excitative property. The iron core per kg obtained when the steel sheet is magnetized to 1.7 tesla (T) at a frequency of 50 Hz, i.e., W.sub.17/50, is used as a numeric value for expressing the iron core property. The magnetic flux density is the maximum governing factor of the iron loss property. In general, the higher the magnetic flux density, the better the iron loss property. In some cases, an increase in the magnetic flux density brings about an increase in the size of the secondary recrystallized grain, so that iron loss becomes poor. In this case, the iron loss property can be improved independently of the grain diameter of the secondary recrystallized grain through the control of a magnetic domain.
The grain oriented electrical steel sheet is produced by developing the so-called "Goss structure" having a &lt;001&gt; axis in the direction of rolling and {110} on the surface of the steel sheet through the occurrence of a secondary recrystallization in the final finish annealing. In order to obtain good magnetic properties, it is necessary to arrange &lt;001&gt;, which is an easily magnetizable axis in the direction of rolling.
Representative examples of the process for producing the above-described monodirectional electro-magnetic steel sheet having a high magnetic flux density include a process disclosed in Japanese Patent Publication No. 15644/1965 by Satoru Taguchi et al. and a process disclosed in Japanese Patent Publication No. 13469/1976 by Takuichi Imanaka et al. In the former, MnS and AlN are used mainly as inhibitors while in the latter, MnS, MnSe, Sb, etc. are used mainly as inhibitors. Therefore, in the current technique, it is inevitable to properly control the size, form and dispersed state of the precipitate that functions as the inhibitor. With respect to MnS, in the current process, MnS is completely dissolved in a solid solution form at the time of heating the slab before hot rolling, and precipitation is conducted at the time of hot rolling. In order to completely dissolve MnS in a solid solution form in an amount necessary for secondary recrystallization, it is necessary to apply a temperature of about 1400.degree. C. This temperature is at least 200.degree. C. above the slab heating temperature of common steel. The slab heating treatment at a high temperature has the following disadvantages.
1) It is necessary to use a high temperature slab heating furnace for exclusive use in directional electrical steel.
2) The energy unit of the heating furnace is high.
3) The amount of molten scale increases, which has a large adverse effect on the operation, such as the necessity of scraping slag.
The above-described problems can be avoided by lowering the slab heating temperature to that used in a common steel. This, however, means that MnS effective as the inhibitor is used in a reduced amount or not used at all, which inevitably renders the secondary recrystallization unstable. For this reason, in order to realize the heating of the slab at a low temperature, it is necessary to strengthen the inhibitor with a precipitate other than MnS to sufficiently inhibit the growth of normal grains during finish annealing. Sulfides and further nitrides, oxides, intergranular precipitation elements, etc. are considered as the above-described inhibitor, and the following are examples of known techniques associated therewith.
Japanese Examined Patent Publication (Kokoku) No. 54-24685 discloses a method wherein the slab heating temperature is made in the range of from 1050 .degree. C. to 1350.degree. C. through the incorporation of an intergranular segregation element, such as As, Bi, Sn or Sb, in the steel. Japanese Unexamined Patent Publication (Kokai) No. 52-24116 discloses a method wherein the slab heating temperature is made in the range of from 1100 .degree. C. to 1260.degree. C. through the incorporation of a nitride forming element, such as Zr, Ti, B, Nb, Ta, V, Cr or Mo, in addition to Al in the steel. Japanese Unexamined Patent Publication (Kokai) No. 57-158322 discloses a method wherein the heating of a slab at a low temperature is made possible through the lowering of the Mn content so as to have an Mn/S ratio of 2.5 or less and, at the same time, the secondary recrystallization is stabilized through the addition of Cu. Further, a technique wherein the strengthening of the inhibitor is combined with an improvement in the metallic structure has also been disclosed. Specifically, in Japanese Unexamined Patent Publication (Kokai) No. 57-89433, the heating of the slab at a low temperature of 1100 .degree. C. to 1250.degree. C. is made possible through a combination of the addition of Mn and an additional element, such as S, Se, Sb, Bi, Pb, Sn or B, with the percentage columnar crystal and the draft in the secondary cold rolling of the slab. Further, Japanese Unexamined Patent Publication (Kokai) No. 59-190324 discloses a method of stabilizing the secondary recrystallization which comprises providing an inhibitor composed mainly of S or Se and Al and B and nitrogen and subjecting the inhibitor to pulse annealing at the time of the primary recrystallization annealing after cold rolling. Thus, a great effort has hitherto been made to enable the slab to be heated at a low temperature in the production of grain oriented electrical steel sheet.
The above-described Japanese Unexamined Patent Publication No. 59-56522 discloses that a slab can be heated at a low temperature when the contents of Mn and S are 0.08 to 0.45 and 0.007% or less, respectively. This method has eliminated the problem of occurrence of a linear secondary crystallization defect of a product attributable to the coarsening of slab grains during heating of the slab at a high temperature.
In the production of a grain oriented electrical steel sheet, annealing of a hot rolled sheet is usually conducted after the hot rolling for the purpose of conducting heterogenization of the structure, precipitation, etc. For example, in the process wherein the inhibitor is composed mainly of AlN, as described in Japanese Examined Patent Publication (Kokoku) No. 23820/1971, the inhibitor is regulated through the precipitation of AlN in the annealing of a hot rolled sheet.
The grain oriented electrical steel sheet is usually produced through main steps such as casting-hot rolling-annealing-cold rolling-decarburization annealing-finish annealing. In this process, a great deal of energy is required, and the production cost is unfavorably higher than that of the common steel manufacturing process, etc.
In recent years, there has been a reconsideration of the above-described manufacturing steps, which consume a large amount of energy, and the simplification and omission of some of the steps and the reduction of energy have been demanded. In order to meet the above-described demand, with respect to a process wherein the inhibitor is mainly composed of AlN, a proposal has been made for the replacement of the precipitation of AlN in the annealing of a hot rolled sheet at a high temperature after the hot rolling (see Japanese Examined Patent Publication (Kokoku) No. 59-45730). In this method, the magnetic properties can be ensured to some extent despite the omission of the annealing of a hot rolled sheet. In the usual method wherein the steel is taken up in a coil form in an amount of 5 to 20 tons, there occurs a difference in the heat history between places within the coil during the step of cooling. This inevitably renders the precipitation of AlN heterogeneous, so that the final magnetic properties varies from place to place in the coil, resulting in the lowering of the yield.
On the other hand, in the process wherein the inhibitor is composed mainly of MnS, MnSe and Sb, a proposal has been made for a method wherein the occurrence of a linear secondary recrystallization defect of a product is inhibited by taking up a steel strip at or below a temperature determined depending upon the cooling rate of a hot rolled steel strip in a period between the separation from a finishing final stand and the taking-up of the steel strip (see Japanese Unexamined Patent Publication (Kokai) No. 59-50118). This method is a technique for inhibiting the occurrence of a linear secondary recrystallization defect attributable to heating of the slab at a high temperature, and the production of a steel sheet by a single cold rolling process wherein the method that omits the annealing of the hot rolled sheet has not been considered.